This invention relates to a rectification process for the separation of C.sub.2+ hydrocarbon fraction from natural gas under elevated pressure, and in particular to a process wherein the temperature and pressure values employed for rectification are obtained by heat exchange and two-stage engine expansion and wherein the rectification is performed between the two expansion stages.
A process of the above-mentioned type has been described in U.S. Pat. No. 4,274,850. In this process, the essential feature requires that the peak cold for the rectification be produced by engine expansion of the overhead product from the rectification step whereas cold is produced at an intermediate temperature level by engine expansion of the natural gas to be fractionated. Rectification takes place, in the presence of limited amounts of CO.sub.2, at a sufficiently high pressure so as to avoid freezing out carbon dioxide. This process, if performed without the use of external refrigeration, requires a higher initial pressure of the natural gas, e.g., generally at least 5 to 15 bar higher than in the process of the invention, the exact value depending upon the composition of the natural gas.